The total amount of solar energy striking the entire surface of the earth is enormous. However, the energy per unit of area is fairly small and calculated by the following equation, due consideration being given to the revolution of the earth and spherical shape of the earth's surface: solar constant of 2 Cal/cm.sup.2.min .times. 1/4 (efficiency of incidence) .times. 2/3 (the remaining 1/3 is reflected from the surface) .times. 2/3 (the remaining 1/3 is absorbed in the atmosphere). The amount of energy thus calculated comes to approximately 133.3 Kcal/m.sup.2.h.
Consequently, in order to utilize the heat energy radiated from the sun as a heat source, it is necessary to concentrate solar light rays to increase the energy density thereof.
However, in calculating the incident solar energy of a solar boiler, it is not required to take into account the above-mentioned efficiency of incidence (1/4) and the efficiency drop caused by reflection from the surface of the earth (2/3), provided that the average energy per unit of area radiated on the entire surface of the earth is always received at a right angle to the reflector surface. Thus, the incident energy received per unit area of the reflector amounts to 799 Kcal/m.sup.2.h (2 Cal/cm.sup.2.min .times. 2/3). A concave mirror is efficient as a condensing means. However, a large concave mirror is difficult to manufacture and is costly.
In order to overcome this difficulty, a method can be considered in which small flat mirrors are placed on a large concave surface to concentrate solar rays received by the mirrors. However, this method is not efficient as an energy concentrating means.
A method can also be considered which utilizes concave spherical mirrors or curved mirrors in place of the small flat mirrors in order to concentrate solar rays on a focal point to increase the energy density. This method is also deficient because generally a concave mirror, regardless of whether it is spherical or parabolic, has a tendency to scatter incident rays projected at an angle to the optical axis.
To gain a better understanding of the scattered focal length, reference is made to FIG. 1 in which an incident ray projected at an angle to the optical axis connecting the center of curvature A and focal point B of a curved mirror reflects from the surface of the curved mirror and does not focus at a point on the optical axis, but is scattered to points outside of the length thereof.
Accordingly, even if a heating surface screen of a solar boiler is positioned at the focal point of such a mirror, the solar rays which are not parallel to the optical axis reach the surface of the boiler before or after they are focused, thus making it difficult to concentrate the solar energy.
As can be understood from the unit Cal/cm. sec. C.degree. of heat conductivity in a heated metal body, the larger the temperature difference between the two opposite surfaces, the more efficient the heat transfer. Therefore, the more concentrated the solar energy and the higher the temperature of the solar rays received at the heating surface, the better the solar boiler efficiency, and the scattered rays impair the function of the solar boiler.